A) Field of the Invention
The present invention relates to a semiconductor device with fuses and its manufacture method, and more particularly to techniques regarding fuse elements used in a trimming circuit or a redundancy circuit of a semiconductor integrated circuit.
B) Description of the Related Art
A trimming circuit and a redundancy circuit are often formed in a semiconductor integrated circuit. If a fuse circuit having fuse elements is used as a trimming circuit or a redundancy circuit, a trimming process and the like can be performed during or after the manufacture of a semiconductor integrated circuit so that the characteristics of the circuit can be improved as much as possible.
Japanese Patent Laid-open Publication HEI-7-307389 discloses in FIG. 1 a circuit having a plurality of parallel connections of a serial connection of a fuse element and a MOS transistor. It discloses that a current drive ability necessary for obtaining a breakdown current for breaking down a fuse element is given by a function of the gate width W of a selection transistor:ID=μCαx(W/L)×(½)×(VGS−Vy)2where ID is a drain current of a selection transistor in a saturation region, and p is a mobility of carriers. Cαx is a gate capacitance of the selection transistor, W is a gate width and L is a gate length. VGS is a gate-source voltage and VY is a threshold voltage.
If the value ID of a saturation drain current necessary for breaking down a fuse element is known, the gate width W (size) of the transistor capable of breaking down the fuse element can be estimated from the above-described equation. This analysis adopts the assumption that the saturation current of a MOS transistor is used for breaking down a fuse.
In order to melt and break down a fuse element, it is necessary to flow current through the fuse element and heat it to a temperature over the melting point thereof. For example, if single crystal silicon or polysilicon is used as the material of a fuse element, a relatively large current is required because the melting point of silicon is as high as about 1420° C. It is therefore necessary to make large the size of a selection transistor, which hinders high integration of device elements. According to the above-described Publication, a bipolar transistor having a high current drive ability is used as a selection transistor to obtain a large current.
Most of recent integrated circuits are MOS type ICs using MOS FETs as fundamental device elements. If a bipolar transistor is required to be formed in a MOS type IC, the element structure becomes complicated and additional processes are necessary.